Breast tissue is composed of fibrous and glandular (fibroglandular) and fatty tissue, where fibroglandular tissue radiologically appears dense on X-ray mammograms and fatty tissue appears lucent. In this context, the term mammographic density, often called breast density, has been used to refer to an estimate of the relative proportion of area that the fibroglandular tissue occupies in the breast tissue as presented in a mammogram.
Women with extremely high mammographic density can have four- to six-times the risk of breast cancer relative to women with predominantly fatty breasts; this may be accounted for by an etiologic effect and/or by a masking effect.
The etiologic effect is reflected in the fact that breast cancers predominantly develop in the epithelial cells that line the ducts of the breast and high mammographic density, which reflects breast composition of predominantly fibrous and glandular tissue, may therefore indicate an increased likelihood of developing breast cancer. The masking effect results from the increased difficulty, and therefore decreased sensitivity, of detecting underlying lesions in mammographically dense regions of a digital mammogram as compared to detecting lesions in fatty regions.
Mammographic density is of particular interest because, unlike most other non-modifiable risk factors (such as age and family history), breast density may be potentially modifiable by therapeutic interventions. An increase in mammographic density over time may be an indicator of elevated breast cancer risk and it has been postulated that a reduction in breast density over time may be related to a decrease in breast cancer risk. Furthermore, the inclusion of mammographic density with other known risk factors may add predictive value in breast cancer risk models and improve individual breast cancer risk predictions.
Breast density has been assessed subjectively using various approaches including categorical scales, Visual Analogue Scales, and semi-automated threshold-based algorithms to describe breast composition in terms of mammographic density.
One such subjective approach is the American College of Radiology (ACR) Breast Imaging Reporting and Data System (BI-RADS) breast composition assessment scale. This scale has been described in numerous studies relating the appearance of mammographic density to breast cancer risk. The ACR BI-RADS scale describes four categories of mammographic breast density and breast composition which radiologists are recommended to use in the evaluation of mammographic density. In some states in the US, the summary of the mammogram report that is sent to patients (sometimes called the lay summary) must contain information about breast density. This information may be worded in lay language instead of using the BI-RADS density lexicon and women whose mammograms show heterogeneously or extremely dense breasts may be told that they have “dense breasts”. A major shortcoming of the ACR BI-RADS scale for reporting breast density is that it is reader dependent; it is a subjective estimate of breast density that may be biased and not reliably reproducible.
While the ACR recommends the use of their BI-RADS scale, they acknowledge that many radiologists will use alternative scales for classification of breast composition from X-ray mammograms such as Wolfe grades, Boyd's Six Category Classification, Tabar patterns, and variations of the BI-RADS density scale. Irrespective of which scales are used, subjective assessments of breast density suffer from a common shortcoming of not being reliably reproducible.